Search for the Standard Model Higgs boson produced in association with a top quark pair in multilepton final states at $\sqrt{s}$ = 13 TeV with the ATLAS detector

This thesis presents the results of a search for the Standard Model Higgs boson produced in association with a pair of top quarks, $t\bar t H$, in multilepton final states. The search is performed using a dataset corresponding to an integrated luminosity of $36.1$ $\mathrm{fb}^{-1}$ of proton-proton collision data recorded by the ATLAS experiment at a center-of-mass energy of $\sqrt{s}=13~\mathrm{TeV}$ at the Large Hadron Collider in 2015 and 2016. The search targets Higgs boson to $WW^{*}$, $\tau\tau$, and $ZZ^{*}$ decay modes. Seven final states, categorized by the number and flavor of the charged leptons, are analyzed in the search for $t\bar t H$ production. The thesis focuses on the analysis of the final state with exactly two same-charge light leptons and one hadronically decaying tau lepton, $2\ell\mathrm{SS1}\tau_\mathrm{had}$. The multivariate technique boosted decision tree is used to discriminate the $t\bar t H$ signal from backgrounds, the latter being dominated by $t \bar{t}$ production with leptons from heavy-flavor hadron decays. A data-driven method is used to estimate the reducible background events. A profile likelihood fit is performed to extract the ratio of the measured $t\bar t H$ signal cross-section to the Standard Model prediction $(\mu_{t\bar t H})$. The observed best-fit value of $\mu_{t\bar t H}$ is $3.5^{+1.7}_{-1.3}$, with an observed (expected) significance of $3.4$ ($1.1$) standard deviations. A cross-check analysis using a simple cut-and-count categorization is also reported. The combination of all seven $t\bar t H$ multilepton final states gives the best-fit value of the $t\bar t H$ signal strength of $\mu_{t\bar t H} = 1.6^{+0.5}_{-0.4}$. An excess of events over the expected Standard Model background is found with an observed (expected) significance of $4.1$ ($2.8$) standard deviations. The cross section for $t\bar t H$ production corresponding to the best-fit value of the signal strength is $\sigma(t\bar t H) = 790^{+230}_{-210}$ fb, as compared to the Standard Model prediction of $507^{+35}_{-50}$ fb. Furthermore, the performance of the ATLAS tau trigger is presented based on the 2015 proton-proton collision data recorded by the ATLAS experiment at $\sqrt{s}=13~\mathrm{TeV}$. A study of the Medipix-$2$ silicon pixel devices used as a luminosity monitoring system is also presented for the data taken in 2012 at $\sqrt{s}=8~\mathrm{TeV}$.